[CVE-2019-5418] File Content Disclosure in Action View

[Aaron Patterson]

There is a possible file content disclosure vulnerability in Action View. This
vulnerability has been assigned the CVE identifier CVE-2019-5418.

Versions Affected: All.
Not affected: None.
Fixed Versions: 6.0.0.beta3, 5.2.2.1, 5.1.6.2, 5.0.7.2, 4.2.11.1

Impact
------
There is a possible file content disclosure vulnerability in Action View.
Specially crafted accept headers in combination with calls to `render file:`
can cause arbitrary files on the target server to be rendered, disclosing the
file contents.

The impact is limited to calls to `render` which render file contents without
a specified accept format. Impacted code in a controller looks something like
this:

```
class UserController < ApplicationController
def index
render file: "#{Rails.root}/some/file"
end
end
```

Rendering templates as opposed to files is not impacted by this vulnerability.

All users running an affected release should either upgrade or use one of the
workarounds immediately.

Releases
--------
The 6.0.0.beta3, 5.2.2.1, 5.1.6.2, 5.0.7.2, and 4.2.11.1 releases are
available at the normal locations.

Workarounds
-----------
This vulnerability can be mitigated by specifying a format for file rendering,
like this:

```
class UserController < ApplicationController
def index
render file: "#{Rails.root}/some/file", formats: [:html]
end
end
```

In summary, impacted calls to `render` look like this:

```
render file: "#{Rails.root}/some/file"
```

The vulnerability can be mitigated by changing to this:

```
render file: "#{Rails.root}/some/file", formats: [:html]
```

Other calls to `render` are not impacted.

Alternatively, the following monkey patch can be applied in an initializer:

```
$ cat config/initializers/formats_filter.rb
# frozen_string_literal: true

ActionDispatch::Request.prepend(Module.new do
def formats
super().select do |format|
format.symbol || format.ref == "*/*"
end
end
end)
```

Patches
-------
To aid users who aren't able to upgrade immediately we have provided patches for
the two supported release series. They are in git-am format and consist of a
single changeset.

* 6-0-action-view-file-disclosure.patch - Patch for 6.0 series
* 5-2-action-view-file-disclosure.patch - Patch for 5.2 series
* 5-1-action-view-file-disclosure.patch - Patch for 5.1 series
* 5-0-action-view-file-disclosure.patch - Patch for 5.0 series
* 4-2-action-view-file-disclosure.patch - Patch for 4.2 series

Please note that only the 5.2.x, 5.1.x, 5.0.x, and 4.2.x series are supported
at present. Users of earlier unsupported releases are advised to upgrade as
soon as possible as we cannot guarantee the continued availability of security
fixes for unsupported releases.

Also note that the patches for this vulnerability are the same as CVE-2019-5419.

Credits
-------
Thanks to John Hawthorn <jo...@hawthorn.email> of GitHub

--
Aaron Patterson
http://tenderlovemaking.com/